1. Technical Field
The present invention relates to an optical position measuring instrument.
2. Background Information
With regard to optical position measuring instruments for detecting relative motions of the scale and scanning unit along curved measurement directions, a distinction can be made between two fundamental types:
a) Optical position measuring instruments with scales in the form of radial graduations disposed on graduated disks;
b) Optical position measuring instruments with scales in the form of drum graduations disposed on graduation drums.
In the first optical position measuring instruments mentioned, which have a graduated disk with a fine radial graduation as a scale, the mounting tolerances of the scale relative to the scanning unit are usually extremely narrow. This is due to the strong signal dropoff, which results at even small radial, tangential or longitudinal deviations in position of the radial graduation from the desired installation location, because of the attendant wave front deformations in the partial beams involved in the signal generation, which are brought into interfering superposition. The radially varying grating constants of the radial graduation cause major wave front deformations. This means that the wave fronts of the partial beams diffracted by the radial graduation sometimes have considerable deviations from planar wave fronts.
Similar problems arise in optical position measuring instruments of the second category as well, in which the scale is disposed as a so-called drum graduation on the outer circumference of a rotating drum or rotating cylinder. Here, the curved drum graduation again causes a distortion of the wave fronts in the partial beams which are used for signal generation.
Such wave front deformations also already result in the ideal mounting location of the scale and will hereinafter be called nominal wave front deformations. If the mounting location is not ideal, additional tolerance-caused wave front deformations occur. The various wave front deformations that occur in the partial beams used for signal generation are thus definitively responsible for the signal dip mentioned at the outset in the position signals generated. The consequence is markedly poorer signal quality of such optical position measuring instruments.
In high-resolution optical position measuring instruments for detecting linear displacement motions of the scale and the scanning unit, it is known to use retroreflectors in the form of triple prisms; as an example, see European Patent Disclosure EP 387 520 A2. In the scanning beam path proposed therein, a collimated beam from a laser light source is diffracted at the linear grating of the scale into partial beams of a +1st and −1st order of diffraction. Next, the partial beams, by one or more retroreflectors in the form of triple prisms, are redirected toward the linear grating of the scale. After a further diffraction at the scale, the two partial beams are made to interfere with one another at a superposition location. By the use of the one or more retroreflectors embodied as triple prisms, it is ensured that even if the scale is arbitrarily tilted relative to the scanning unit, the two partial beams, after the second diffraction at the linear grating of the scale, maintain their direction. No wave front tilting of the interfering partial beams then occurs. As a result, there is maximum interference contrast in the overlapping range of the interfering partial beams. In this manner, fundamentally wide mounting tolerances are attainable, even with extremely fine graduation periods of the scale and large scanning areas, that is, large beam cross sections at the location of the scale. Basically, however, the good properties of such optical position measuring instruments are based on the condition that the wave fronts of the partial beams remain as planar as possible, both after the diffraction at the linear grating of the scale and after the reflection at the retroreflector. As a result, the wave front tilting caused by tilting of the scale is ideally compensated for by the retroreflectors used.
If high-resolution optical position measuring instruments are now to be used for detecting relative motions of the scale and scanning unit along curved measurement directions, or in other words, if systems with radial grating graduations or drum graduations are used in conjunction with retroreflectors, certain problems arise. In U.S. Pat. No. 5,442,172, the entire contents of which are incorporated herein by reference, these problems are analyzed and supposed solutions to them are proposed. For instance, according to this reference, the influence of wave front deformations that impairs the signal quality is reduced because an ideal reflector unit is proposed. This unit includes a combination of a spherical lens and a roof prism, which is disposed in the focal plane of the lens. However, a more-precise analysis of the proposed scanning optics shows that a significant signal dropoff still occurs if the scale and the scanning unit are incorrectly adjusted. A further factor is that in the proposed ideal retroreflector unit, the beam focus is located at the roof of the roof prism, which must therefore be manufactured absolutely without flaws in that area. In that area, there must be absolutely no nonhomogeneities, such as inclusions, dirt, or stepped edges. Because of the stringent demands made in terms of the production of such a component, this component is thus extremely expensive.